thank you for the report. You are right: when PDF24 splits a PDF, each part is put together from the pages, and the layer definitions of the original file are currently not taken over into the parts. This applies to the PDF24 Toolbox and to the online tool https://tools.pdf24.org/en/split-pdf alike.
We will add it to our list so that the parts keep their layers. We will post here when it is available.